Focus is the one area that lenses outperform the human eye. Lenses come in a wide variety of focal lengths, allowing them to “see” objects that are very large and very far away, like galaxies and stars, or objects that are very small and very nearby, like bacteria and DNA. The average zoom lens on a video camera doesn’t have that kind of range, but it’s still far superior to the human eye. Humans have only one focal length to deal with at any given moment, and that, regrettably, tends to shrink over time.
We do have one advantage. Focus, like exposure, is dependent on the iris’ ability to widen and narrow. The smaller the aperture, the farther away and closer the eye or lens will be able to focus. The one advantage that our eyes have over lenses, well most lenses, is in low light. The iris of the lens may be narrow enough to maintain focus, but too narrow to achieve sufficient exposure. This happens to humans as well as cameras, but eyes have the nifty ability to adjust to the darkness, not to all darkness of course. We can’t make anything out in the pitch black, but we do a better job than most lenses. We don’t need a night shot option, like our video cameras, to make out that shadowy figure, in the dark, at the stop of the stairs.
How Focus Works
The area of your frame, where objects are in sharpest focus is called the focal point. When the camera is focused correctly, the subject of your shot and the focal point occupy the same position or are the same distance from the lens. Focus reduces gradually in front of and behind the focal point. If the subject and focal point are not on the same plane, the focus will be soft. The zone in front of and behind the focal point, where objects are more or less in focus and not too noticeably blurred is called the depth of field.
You can determine how much of the frame will be in focus and how much will be blurred by controlling the depth of field. There are three factors that determine the focal depth of your shot. The first and most important is the size of the aperture or iris. A narrow iris gives you a long depth of field. A wide iris gives you a shallow depth of field. The size of the iris is measured in f-stops. Counter intuitively, the larger the f-stop, the smaller the iris. Typically f-stop numbers range in size from f/1.4, which is the largest opening, to f/22, which is the smallest.
The next is the amount of light in the scene. Amount of light is important, because it determines how you set your exposure, i.e., how wide your iris is (which ultimately determines depth of field) and how fast you make your shutter speed. Shutter speed can have an indirect affect on depth of field. For example, if you want shallow focal depth, so that objects in the foreground and background of your shot will be blurred, but your shooting outside on a sunny day, you could change the shutter speed so that the exposure time will be shorter. This will allow you to open your aperture a wider and reduce the depth of field. A neutral density filter is another option for reducing the amount of light entering the lens, so you can open up the iris.
And finally, depth of field is influenced by the position of your zoom. When the lens is zoomed out all the way depth of field is longer than it is when the lens is zoomed in. So, if you want to keep your depth of field as long as possible while your shooting, keep the camera zoomed out and simply move closer to your subject for close ups and medium shorts rather than zooming in.
Manual Focus Variations
Manual focus can vary dramatically from camera to camera. Most units have a button somewhere on the camcorder that will toggle auto-focus on and off. Once your camera is set to manual focus, you can make adjustments using the camera’s focus ring, dial, slider, LCD or Joystick, whatever device the designers of your unit have chosen. A focus ring around the lens of the camera is the easiest to control. Unfortunately, most consumer-grade video cameras employ a much smaller dial or slider along the side or back of the camera. Controlling these is trickier than a focus ring. Some of the latest models employ a touch-screen system on the LCD or a joystick at the back of the camera. These require a delicate touch, are not an improvement on a dial or slider, and are certainly more difficult to operate than a focus ring. Whatever method of manual control your camera uses, it’s best to have some practice using it before you get into a shoot that’s important to you.
When to Use Manual Focus
Like automatic exposure, auto-focus mode is pretty reliable under most circumstances. Ideally, you’re shooting under well-lit conditions. In this case, you’re aperture opening is relatively small, and the depth of field is relatively wide. In these conditions, automatic focus usually works pretty well and often better than even an experienced cameraperson. If you’re working in low light, a crowd or busy scene, then the automatic focus may get fooled and you’ll need to focus manually. Manual focus also allows you to have more creative control over your shot. When the depth of field is shallow you can experiment with selective and rack focus, which along with good composition, can help you establish a distinct style or mood.
Whenever your camera and the subject are stationary, during an interview or a staged event for instance, it’s a good idea to do a critical focus. What this does is clearly define the focal point at your intended subject, which then allows you to zoom in and out without the focus going soft.
The procedure is simple. Zoom all the way in on your subject, usually on a person’s eyes. Place your camcorder in manual focus mode. Adjust the focus for the sharpest image. Then zoom back out to frame the shot, as you desire.
Critical focus allows you to shoot a subject that is relatively stationary, like an interview or a speech, without thinking about the focus at all. Once it’s set, it’s set. It’s also a good opportunity to practice your zooms. Keeping your zooms smooth and steady takes practice. Fast zooms in or out are always noticeable, and rarely appropriate for an interview. However, a slow zoom in from a medium shot to a close-up or from a close-up to a medium shot can be a nice effect and will give you more options when it comes time to edit.
Crowded or Busy Scenes
When in automatic mode, the camera sets the focus to be sharp for the most distinct object in or near the center of the frame. If the object in the center of the frame changes, which often happens when shooting in a crowd, then the focus shifts. Something or someone moving in front of the subject of your shot, will steal the focus as they pass, only to have the focus shift back again to your subject once the usurper has left the frame. Even if your subject remains the center of attention, moving too close or too far away from the camera can force a change in focal point that you or the camera’s automatic focus may not be able to keep up with. These focus shifts can be obvious and don’t always happen quickly. This effect, described as hunting, can be really distracting if left unchecked.
So, what do you do? The answer depends on how and what you’re shooting. If you and the camera are mobile and following a single subject – say for instance, you’re following the mayor as he meets and greets his constituents at a community function–then putting the camera in manual focus, setting the focal point for a few feet ahead of you, should do the trick, so long as you keep the distance between you and the mayor roughly that same few feet. People passing in front of you might not be in focus, but they aren’t the subject of your shot.
When you turn the camera around to shoot the mayor’s constituents, it’s best to take an approach that’s similar to a still photographer. Don’t keep the camera rolling continuously. Take the time to decide on a subject and set up your shot. Zoom in if you must, but don’t do it while your shooting. You don’t want to manually shift your focus while your shooting either. Set the focus before you begin recording. Think of each shot as a separate still image, then shoot for at least ten to fifteen seconds. If you follow this procedure, the focus on your subject will remain sharp, regardless of any interference. Your composition will be tight and you’re shot length will increase your editing options.
On the other hand, if you’re shooting a party or a demonstration and you want your audience to feel as if they were right there with you, then allowing the camera to roll as you move from one subject to the next might be the way to go. If the scene is well lit and the depth of field is wide, auto-focus may be able to keep up with the shifts in focus. Following the action and remaining in focus manually tends not to work so well, especially, if you’re dealing with a touch screen focus system or a joystick. You can help auto-focus do it’s job by staying zoomed out.
Whether you’re in auto-focus or manual, you should avoid zooms while shooting. The unrehearsed zoom is always a risk. When the camera is zoomed out the depth of field will only get wider if you adjust the iris to a smaller size. If you don’t know how much focal depth you’ve got, you don’t want to shrink it unnecessarily. But this is exactly what happens when the camera zooms in. The depth of field shrinks, throwing objects in the foreground and background out of focus. The subject of your shot may have been within the depth of field before the zoom, but if the subject is not in the same plane as the focal point, it may go out of focus as the depth of field decreases during the zoom. For this reason, zooms should be reserved for stationary subjects. If neither you nor the subjects in your shot are stationary, then the chances of losing focus during the zoom increase considerably. If you’re moving through a crowd and you are fortunate enough to have a healthy amount of light and a fairly wide depth of field, don’t blow your advantage by zooming in and out. If you want to help maintain focus, keep the shot as wide as possible. If you want close ups or medium shots, simply move the camera closer to the subject.
Focusing in Low Light
Because the aperture in extremely low light is open as wide as it will go, the depth of field will be very shallow. The auto-focus will do it’s best to keep whatever is in the center of the frame in focus, but as objects in the center of the frame change or move about, the focus will shift as well. This is called hunting and it often happens in crowded or busy scenes, but if the light is low, the auto-focus will hunt for a target even when there’s very little movement in the frame. In very low light, colors lose their vibrancy and edges melt together. If everything looks like gray murky shadows to the camera, the auto-focus it has a hard time locking on to anything, whether it’s in the middle of the frame or not.
If you can find a way to add light to your subject, do so, as this will increase your focal depth making holding focus easier. If the camera and the subject are both relatively stable, as when shooting a speaker or even a recital, then performing a critical focus should help. Another option is to place your subject in the center of the frame and zoom in for a tighter shot. You’ll sacrifice some of your focal depth this way, but if someone’s head is filling the frame, the camera is less likely to be confused about what it’s supposed to be focusing on. Even in the auto-focus mode, a subject that’s not moving around too much and filling the majority of the frame should remain in focus. This won’t work if your shooting a group performance, but works fine for speeches and soloists. A better option, if you can manage it, is to keep your shot wide and move in close for a tighter shot. This way, you can achieve the same clarity without sacrificing focal depth. It also gives your subject a little bit more leeway to move around in the frame.
If you have the combined problem of a lot of motion and low light, then you do not have the option of using auto-focus. Manual focus will stop the auto focus from “hunting” for the right subject in the dim light. If you’re working with a very low-end camera that doesn’t offer you manual focus control, you’ll need to take further steps. First of all, you’ll need to keep the camera very still, not just steady, still like a still camera. This is because every time you move the camera, the object in the center of the frame, which the camera is trying to focus on, will move and then the focus will shift. Lock off your shot, preferably on a tripod or monopod. The focus will shift just as much if the subject is moving as it will if the camera is moving. So, once you’ve locked off the camera, you’ll need to try and keep the subject steady as well. Don’t think about following the action when the light is too low. Find action that’s sitting still and just focus on that. Keeping the camera close to the subject but zoomed out all the way, will help you maintain focus as well.
If there isn’t even enough light for your camera to hold focus and you can’t boost the light, then you can experiment with night shot or changing the shutter speed. This may cause a trailing effect. But as this is obviously an exposure problem as well as a focus problem, you may have no choice.
Selective and Rack Focus
When in automatic mode, most cameras set the focus to be sharp for the most distinct object in or near the center of the frame. Usually, that includes the intended subject of your shot, but not always. What do you do if the subject of your shot is not in the center of the frame, or if you don’t want them to be in the center of the frame? You may determine that your composition is more meaningful or striking if the subject of your shot is along the side or near the bottom of the frame. In this case, put the camera on manual and change the focus until your subject is clear. Or you can put object you want in focus in the center of the frame allowing the auto-focus to make the adjustment, then switch to manual (locking the focal point at that distance) then reframe as you wish.
Purposefully blurring objects in the background and foreground of the frame clues the audience into what they should be focusing on without removing other elements that may also be relevant. Selective and rack focus is only possible if your depth of field is shallow, to achieve that it helps a great deal if you have some control over the light. You may need to reduce the amount of light in your scene in order to widen the aperture without over-exposing your shot. If you don’t have control over the lights in your shot, if you’re outside for instance, you might consider using a neutral density filter. If the ND filter doesn’t reduce the light enough or if you don’t have an ND filter, you can try zooming in on your subject, and moving your camera, this will reduce your depth of field. If you zoom in and widen the aperture, then you can really reduce your depth of field. A simpler option is to speed up the shutter so you can widen the aperture. This option works best if there’s not too much movement in the shot, as faster shutter speeds can make your action look jerky.